ИСТИНА

Interpretation of the time-domain electromagnetic sounding (TDEM) data suggests using different types of inverse problem. In most cases, it is one-dimensional (1D) inversion. There is a lot of software, based on 1D inversion, which can be divided into two groups: 1) automatic Occam’s inversion, where multilayered model is formed with or without using a-priori information and 2) inversion based on thick-layered model mostly using a-priori information. There is much less software for 3D inversion, because it is more time-consuming. In some cases, such as paleovalley exploration, where lateral variations of resistivity are intensive (so, the media is not horizontally layered), but the anomalies are elongated in one direction, 2D inversion could be very useful. Another advantage is that 2D inversion uses the data along profiles, when using 3D inversion requires areal survey. The features and benefits of the two-dimensional inversion algorithm implemented in the program ZondTEM2D are presented. Synthetic TDEM data was calculated for models, based on the results of joint inversion EM (VES, TDEM and AMT) data along two profiles across paleovalley in Kaluga region (Russia) using 3D forward procedure in GeoEM software (Maria Persova, Novosibirsk, Russia). Field TDEM data was acquired by students of Moscow State University during their summer field courses using TEMFast-48 equipment (AEMR Ltd.). Both transmitter and receiver loops were square (50 m x 50 m), time range was from 0.01 to 1 msec, the distance between sites was 35 or 70 m, the length of profiles was 6 and 7.5 km, the distance between profiles was 3 – 4 km. Both synthetic and field data inversion showed the possibility of reconstruction of the resistivity for models containing the objects of “paleovalley type” ‒ high-resistivity lenses within the horizontally layered host medium.